Technical Abstract:
Aeration of ponds when dissolved oxygen (DO) concentrations are low is the principal management tool that allows for higher feeding rates, increased production, and decreased cost per unit fish produced. Recent research conducted at the USDA-ARS Warmwater Aquaculture Research Unit has shown that feed conversion ratio (FCR) can be improved by raising the minimum DO concentration to 1.6 mg/L. Catfish will also consume more feed and grow faster if the minimum DO concentration is not allowed to drop below 3.0 mg/L. This faster growth results in a shorter production period, better survival, improved FCR, and reduces economic risk.
Traditional 10-hp electric paddlewheel aerators have proven to be the most efficient aerator for routine aeration of commercial catfish ponds, although they do have a performance limitation. While they efficiently transfer large quantities of oxygen to the water (measured in pounds of oxygen transferred per horsepower-hour), they also move a great volume of water. The end result is that the concentration of oxygen in the water is only increased a small amount (perhaps only 1.0 mg/L) by aeration. During routine aeration this is sufficient, but when the pond DO concentration is very low from a bloom die-off, this can result in problems ranging from reduced feed consumption and growth to complete fish kills. Adding more aerators (tractor-powered sidewinders or paddlewheels) increases the current in the pond, but in many cases can’t overcome the tremendous oxygen demand resulting from the lack of photosynthesis and decomposing bloom. The fish can maintain their position at the aerators for a while, but if pond conditions do not improve quickly, they eventually become exhausted and drift away from the aerators into water devoid of oxygen, resulting in a major fish kill. Unfortunately, most farmers have experienced this tragic loss.
Subsurface aerators are an option to paddlewheel aerators. However, fish ponds are seldom more than 5 feet deep and technologies such as diffused-air systems require greater depths to operate efficiently. A new aerator developed by researchers in Stoneville, known as the “Power Tube Airlift” (PTA), has the ability to use low pressure air to increase the transfer rate and efficiency to a level comparable to paddlewheel aerators. The PTA was also designed to move less water than a paddlewheel aerator but enough water to create a zone of elevated DO large enough for a great biomass of fish.
Two commercial-size PTAs were fabricated and installed in an 8 acre traditional catfish pond on-site. Each PTA consisted of a 3.0-ft diameter intake, a 4.0-ft diameter outflow, and a blower powered by a 10-hp, 3-phase electric motor (Figure 1). A main air supply line was plumbed into a sparger assembly positioned at a 25 ft water depth on the outflow side of each unit. The high-volume discharge through the sparger (larger bubbles) provided “lift” to move the water through the PTA. These larger bubbles increased water pumping efficiency. Approximately 10-15% of the total air flow was diverted to a diffuser grid (smaller bubbles) placed at an 8 ft water depth on the intake side. These smaller bubbles increased oxygen transfer to the water. For a preliminary field test in 2012, hybrid catfish stockers were conservatively stocked at about 2,100/ac (600 lb/1,000) for food fish production. Dissolved oxygen was continuously monitored with an automated monitoring system that controlled the operation of the PTAs throughout the study.
The two PTA aerators were able to maintain DO concentrations of about 2.3 mg/L greater than the outside area of the pond. For example, during a 2-week period in July 2012 the average morning DO concentration for the outside area was 1.0 mg/L while the aeration zone was maintained at 3.3 mg/L. The aeration zone was approximately 0.6 acres or about 7% of the total pond volume. A